A conventional video switching device will be described with reference to FIGS. 3 to 8.
FIG. 3 is a block diagram for explaining the conventional video switching device.
FIGS. 4A and 4B are a timing chart for explaining the operation shown in FIG. 3.
Referring to FIG. 3, a video switching device 300 includes a video switching unit 310 and an encoding device 320.
The video switching unit 310 includes a control detection unit (V-BL, Vertical Blanking response) 113 and a camera video switching unit 112. The video switching unit 310 has two input terminals for inputting video signals VID1 (Video1) and VID2 (Video2), and an output terminal for outputting a video signal SI-VID (Selected-Video).
When a control signal (serial) SW-C is inputted from the outside, the control detection unit (V-BL response) 113 outputs a camera switching instruction signal SW-VBL for switching a video signal during V-BL (Vertical Blanking, vertical blanking interval) to the camera video switching unit 112.
The camera video switching unit 112 outputs a video signal VID1 or VID2 based on the camera switching instruction signal SW-VBL. For example, as shown in FIGS. 4A and 4B, the camera video switching unit 112 outputs VID1 when the camera switching instruction signal SW-VBL of a low-level is outputted, and the camera video switching unit 112 outputs VID2 when the camera switching instruction signal SW-VBL of a high-level.
The video signals VID1 and VID2 are outputted from, e.g., cameras 101 and 102, and the like.
The encoding device 320 includes a control unit 321, an encoding unit 130, and an SG (Sync Generator) unit 140.
The control unit 321 inputs a bit rate control signal and SUM, and outputs a comp control signal and an I/P0 control signal.
The I/P0 control signal outputted from the control unit 321 is a signal that changes while shifting a target range to a lower stage from an upper part of a screen for each frame.
The encoding unit 130 for performing image compression includes an I (Intra-coded Picture) processing unit 131, a P (Predictive-coded Picture) processing unit 132, a selection unit 133, a buffer memory unit 134, and a decoding unit 135.
In the encoding unit 130, the I processing unit 131 generates compressed data I-CD from the inputted video signal SI-VID; the P processing unit 132 generates compressed data P-CD from the inputted video signal SI-VID; the selection unit 133 selects the compressed data I-CD or P-CD and outputs the selected data S-CD to the buffer memory unit 134; and the buffer memory unit 134 outputs compressed data. Further, the decoding unit 135 decodes the selected data S-CD and outputs the decoded video signal V-DEM to the P processing unit 132.
FIG. 6 is a block diagram for explaining the operation of the I processing unit.
The I processing unit 131 includes a converting unit 801, a quantization unit 802, and a Huffman coding unit 803. The converting unit 801 performs, e.g., DCT (Discrete Cosine Transform) conversion on the inputted video signal SI-VID. The quantization unit 802 and the Huffman coding unit 803 create and output the compressed data I-CD.
FIG. 7 is a block diagram for explaining the operation of the P processing unit.
The P processing unit 132 includes a difference unit 904, a converting unit 801, a quantization unit 802, and a Huffman coding unit 803. The difference unit 904 obtains a difference between the video signal SI-VID of the current frame and the video signal V-DEM of the previous frame. The converting unit 801 performs, e.g., DCT (Discrete Cosine Transform) conversion, on the difference video signal. The quantization unit 802 and the Huffman coding unit 803 create and output the compressed P-CD.
In the encoding unit 130, a normal video includes a main picture having high correlation with the previous frame with the same picture as that of the previous frame partially shifted. Therefore, the difference between the previous frame image and the current frame image is obtained (referred to as “P processing”). Then, the difference is encoded and quantized to create compressed data.
The I processing is performed on a part of the video, and the P processing is performed on the other part of the video.
The ordinary video is designed to have a small difference between frames and a smaller amount of data. Particularly, a still picture is substantially the same as that of the previous frame. Therefore, in the case of the still picture, the difference is 0 and the amount of data newly generated by the P processing is substantially 0.
FIGS. 5A and 5B explain the amount of generated data and the image quality of similar images and different images between frames in the I processing and the P processing of images.
FIG. 5A shows the amount of data generated in the case of performing constant quantization. In the I processing, a large amount data is generated in both of similar images and different images. In the P processing, a small amount of data is generated in similar images and a large amount of data is generated in heterogeneous images.
FIG. 5B shows the image quality obtained when the amount of data is constant. In the I processing, the image quality is poor in both of similar images and different images. In the P processing, the image quality is good in the similar images and is poor in the different images.
FIG. 8 explains an image obtained by conventional image switching.
When the fine components outputted from the converting unit 801 are discarded and decoded, the error from the original image difference is increased. As a result, the image reproducibility is decreased and the image quality deteriorates. Such characteristics are shown in FIGS. 5A, 5B and 8.
In FIG. 8, the entire picture was changed in the third frame (c). Therefore, in each P processing, the changes are increased due to different images, and the amount of generated data tends to be increased. Accordingly, the amount of data is decreased by coarse quantization.
In a transition period in which the amount of generated data tends to be increased in each P processing, the amount of data that can be allocated to the I processing is decreased. Therefore, the image quality of the decoded video deteriorates and details of the entire screen are omitted.
In a prior art document, e.g., in Patent Document 1, in a video sending device having an active system and one or more standby systems, a digital video signal is distributed to the active system and the standby systems, and the amount of video data of each video frame in a decoded output of the active system is calculated by a video change detector. The active system is switched to the standby system to output the video data when it is detected that the amount of video data of each video frame is not changed.    Patent Document 1: Japanese Patent Application Publication No. 2007-43520